A
add to sth有”扩建,扩大“的意思.
The added to the house the next year.第二年,他们扩建了房屋.
这里其实是一个定语从句,The exhibition hall,（which was added to many times）展厅被扩建了许多次
C
考点：将来完成时
将来完成时用来表示在将来某一时间以前已经完成或一直持续的动作.经常与before+将来时间或by+将来时间连用,也可与before或by the time短语引导的现在时的从句连用.
你从国外回来的时候,我将已经离开长沙了.
例子：
By the time you get home I will have cleaned the house from top to bottom.
你到家之前我将把房子彻底打扫一遍.